Sunday, January 22, 2006

Current Atmospheric Circulation: Rogue or Linear?

The matters that I discussed on the January 18th posting are still extremely relevant. We are simply at the point where all that can be done is monitor, particularly if and when a sudden amplification of the westerlies across the Asia-PNA sector occur. The details of what the outcome of such kind of event are unclear. However, the most probable solution would be a western Pacific trough, central-east Pacific ridge with a downstream trough across western North America by roughly 7-14 days from now (30 January - 6 February). Brief justification for this possibility is given below.

During the past week the tropical convective forcing became concentrated and very intense centered roughly on the equator/140E (north of Australia). Anomalies less than -90 w/m**2 were observed, and may have been the result of a convectively coupled Kelvin wave merging with convection moving northwest from the south Pacific. This flare-up has already impacted the circulation, initiating, for example, a Rossby wave energy dipersion linked to a couple of recent storm systems across the USA. The stationary tropical forcing has increased in areal coverage the last couple of days, extending from the eastern Indian Ocean to just northeast of New Zealand. MJO activity is nil to very weak (indicies such as the Wheeler phase space plot are not representative), and the atmosphere continues to respond to at least weak La-Nina conditions (equatorial SST anomalies have cooled to less than -1.5C around 140W January 21).

At this time a very fast synoptic wave train moving though the subtropics of the eastern hemisphere is interacting with the tropical convective forcing. Upper tropospheric divergent outflow from this thunderstorm activity is quite impressive, and will pose a difficult predictability problem for all the operational numerical global models. Today and for the past several days all model forecasts have been quite varied, generally suggesting a low amplitude flow with progressive synoptic systems. The CDC ensemble from 0000 UTC 22 January 2006 initial conditions does suggest a large amplitude version of the scenario given in the first paragraph. If the atmosphere responds as "linear thinking" would suggest, the CDC ensemble solution would be reasonable. The latter is given by SDM Stage 1, and, in this case, would project onto negative phases of the PNA, AO and NAO (polar latitude Stratospheric easterlies have already propagated into the Troposphere due to the recent SSW). A transition to SDM Stage 2 would then be expected (ridge closer to west coast of USA going into week 3).

As we all observed during the last 2-3 weeks of December continuing into this month, the atmosphere went "rogue", and the jet expanded all the way from north of India to the west coast of the USA (anomalies in excess of 40 m/s, at times). This event was about as extreme (nonlinear) as anything can get, and it would be interesting to attempt to reproduce this jet in a general circulation model (GCM). As a note, utilized along with our SDM, it would also be nice to run climate models in near real-time. In any event, in regard to making a prediction for days 3-20, I have to go with what linear meteorological thinking tells me. That would be at least a low amplitude version of the circulation predicted by the CDC ensemble. Future model runs must also be carefully monitored.

All of the above suggests a possibility of a sharp weather change for much of the lower 48 states during week 2 (starting next weekend). The western half-two thirds of the country may cool to below normal temperatures due to Arctic air, while the southeast stays warm. Synoptic systems may stay progressive, with perhaps an active storm track with high impact weather from the Pacific Northwest into the Rockies, then northeastward across the Plains. Given tremendous uncertainty, I do not want to give any more details.

For southwest Kansas, given our location, there is not much I offer in regard to real hope for much needed significant precipitation. This upcoming week looks "same old-same old"; above normal temperatures and basically dry. The notion of another weak storm with light precipitation for roughly Thursday appears reasonable. For the period of next weekend through February 6th, temperatures may lower to at least normal, with perhaps an episode or 2 of well below normal temperatures. However, I am still concerned about storm systems being too progressive to give us much precipitation. For locations to our east and north, such as eastern Kansas into Iowa, lots of snow may occur.

Ed Berry

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